Interpretive Summary: The amount of pesticide in soil that is available for movement to the target pest or for movement into surface and ground waters is controlled by the degree of binding of the chemical to the soil and the rate at which it degrades. Binding and degradation of pesticides are usually characterized for pesticides freshly added to soil, however we have shown in previous studies that the length of time the pesticide is in contact with soil, aging, can affect these processes. While there is limited information on binding and degradation of aged pesticide residues, there is less information on bioavailability and degradation of pesticide breakdown products or metabolites. The present study was conducted to determine the changes in distribution between soil bound and solution phases of two metabolites of the insecticide imdiacloprid, imidacloprid-urea and imidacloprid-guanidine, with incubation time. We found that while the pesticide metabolites slowly degraded in soil, the remaining chemical became more tightly bound to the soil. Therefore, the aged residues would be less available for movement to surface and ground waters. These results will be of interest/benefit to scientists developing models to predict pesticide behavior in the environment, who will then be able to take into account increased binding during pesticide aging in soil in the development of mathematical models of pesticide degradation and transport.

Technical Abstract:
Changes in sorption/bioavailability of two metabolites, 1-[(6-chloro-3- pyridinyl)methyl]-2-imidazolidinone (imidacloprid-urea) and 1-[(6-chloro-3- pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine (imidacloprid-guanidine) of the insecticide imidacloprid (1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro- 2-imidazolidinimine) with aging in different soils were determined. Soil moisture was adjusted to -33 kPa and 14C- and analytical-grade imidacloprid-urea and imidacloprid-guanidine were added to the soil at a rate of 1.0 mg# kg-1. Spiked soils were incubated at 25 oC for 8 wks. Replicate soil samples were periodically extracted successively with 0.01 N CaCl2, acetonitrile, and 1 N HCl. Imidacloprid-urea sorption, as indicated by Kd values, was highest in the soil with highest OC content, and increased by an average factor of 2.7 in two soils during the 8-wk incubation period. Imidacloprid-guanidine sorption increased by a factor of f2.3 in the same soils. The increase in sorption was the result of a decrease in the metabolite extractable with CaCl2 (solution phase); the amount of metabolite extractable with acetonitrile and HCl (sorbed phase) did not significantly change with incubation time. It appears the increase in sorption was because the rate of degradation in solution and on labile sites was faster than the rate of desorption from the soil particles. It may have also been due to metabolite diffusion to less accessible or stronger binding sites with time. Regardless of the mechanism, these results are further evidence that increases in sorption during pesticide aging should be taken into account during characterization of the sorption process for mathematical models of pesticide degradation and transport.